Automatic test equipment or ATE is used to test semiconductor or other type devices at various stages of manufacture. An ATE tester generates signals, supplies the signals to a device under test or DUT, and monitors the responses to these signals to evaluate the fitness of the DUT. These signals include DC signals, and time varying signals such as AC, pulsed, or other periodic signals.
To provide these signals with precision, testers employ DC circuitry, and time varying signal circuitry sometimes referred to as pin electronics circuitry. Separate circuits are used because the circuits capable of providing and measuring precision DC signals are not capable of providing and measuring precision high frequency signals. Likewise, precision time varying signal circuits are not able to provide and measure precision DC signals. Thus, the tester must be capable of switching between these two circuits when testing a DUT.
Further, to maintain precise time varying signal characteristics, testers must be capable of performing self calibration. Self calibration includes signal level calibration as well as signal timing calibration. Thus, the tester also must provide this switching for calibration.
FIG. 1 shows a "T" switching circuit commonly used in prior art ATE testers to perform such switching. Relays typically are employed to provide switching. Relay switches facilitate precision testing by providing low closed resistance and low open capacitance. This allows accurate transmission and measurement of signal timing and signal levels over a wide bandwidth and range of signal levels. Thus, with appropriate relay switching, the "T" switching circuit is able to provide low resistance and low capacitance transmission paths between the time varying signal circuitry 10 and the DUT 30, between the DC test/time varying signal calibration circuitry 20 and the DUT 30, as well as between the time varying signal circuitry 10 and the DC test/time varying signal calibration circuitry 20.
Relays, however, have a significant drawback. Relays have a relatively low mean time before failure or MTBF as compared to other tester components. One cause of the low MTBF of relays is polymer build-up on the surface of the relay contacts. Contacts are susceptible to polymer build-up when switched dry rather than under an applied current or voltage. Such polymer build-up increases contact resistance. Moreover, the resistance caused by polymer build-up varies each time the contacts are closed. This is particularly true in relays designed for high bandwidth applications. In such applications, relays having small contacts to provide lower capacitance along the high frequency transmission line also have a reduced spring force, which facilitates resistance variations in polymerized contacts. In testers designed to test devices 125 Mhz-500 Mhz or greater, relays normally having only a fraction of an ohm resistance, can develop several ohms of resistance. This results in each closure of the relay leading to a different resistance value, which affects measurement precision and, consequently, the reliability of the tester. As such, relays contribute to tester down time, slowing production and reducing product margins. To compete in semiconductor and other electronic devices markets, manufacturers require more reliable test equipment.
Solid state switches, on the other hand, generally have orders of magnitude higher MTBF. Solid state switches, however, are not capable of providing the same low resistance and capacitance as relays. FIG. 2 illustrates a comparison of resistance verses capacitance characteristics of solid state switches and relays. Whereas the product of the closed resistance and open capacitance of a high frequency relay can be on the order of 0.07 pF-Ohms, the best commercially available solid state devices provide only about 15-40 pF-Ohms.
As such, although replacing RELAY1, RELAY2, and RELAY3, of FIG. 1 with a solid state switch could improve MTBF, it also impermissibly impairs the capabilities of the tester. This is particularly true in high frequency applications, where the bandwidth of the time varying signal is limited by the capacitance of the time varying transmission channel. Furthermore, increased resistance limits precision of DC signal measurement and of time varying signal calibration.